1. Field of the Invention
The invention relates to an organic electroluminescence device (hereinafter also referred to as organic EL device) which may be effectively utilized as surface light sources for full color displays, backlights, illumination sources and the like, and light source arrays for printers and the like.
2. Related Art
Nowadays, various display devices have been actively researched and developed, and particularly organic EL devices can attain highly intensive light emission with low voltage and are thus attracted as promising display devices.
An organic EL device is composed of a luminescence layer or plural organic layers containing a luminescence layer, and a pair of electrodes into which an organic layer was interposed. The organic EL device is a device wherein an electron injected from a cathode and a hole injected from an anode are recombined in an organic layer, to utilize emission from an exciton formed and/or emission from another exciton molecule formed by energy transfer from the above exciton.
The organic EL device is also characterized in that luminescence of various luminescence colors is possible by mixing plural luminescence colors.
Among luminescence colors, there is particularly high need for white luminescence. White luminescence can be utilized for electrical power saving in generic illumination, in-vehicle displays, and backlights. A color filter may be used to divide white luminescence into blue, green and red pixels or to enable a full-color display.
For example, an organic EL device wherein two or more different luminescence materials are contained in a luminescence layer and at least one of the luminescence materials is an ortho-metalated complex is disclosed (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2001-319780).
However, plural luminescence materials differ in the level of exciton energy in the luminescence layer and are less liable to efficient, simultaneous emission. Moreover, the produced excitons may be quenched by diffusion to adjacent layers, thus causing a problem of reduction in efficiency.
It is disclosed that in the structure of a luminescence layer containing a host material and a blue phosphorescence material having an emission peak wavelength of 500 nm or less, a combination of the host material and the blue phosphorescence material in which the energy difference (T1) between the ground state and the excited triplet state of the host material is in the range of 1.05 times to 1.38 times based on the T1 of the blue phosphorescence material produces high emission efficiency (see, for example, JP-A No. 2002-100476). As the host material, an azole compound having a specific structure is disclosed.
A large excited triplet state energy is necessary for emitting a blue phosphorescence material, and a carbazole compound having a specific structure as a charge transporting material in a luminescence layer has such a large excited triplet state energy. Therefore, a luminescence layer containing a blue phosphorescence material and the carbazole compound is alleged to give a high-efficiency blue phosphorescence emission (see, for example, JP-A No. 2004-273128).
In the structures shown in JP-A No. 2002-100476 and JP-A No. 2004-273128, however, it is still not possible to improve inefficiency attributable to excitons leaking from a luminescence layer to its adjacent layers.